This invention relates to a method of and apparatus for determining the velocity of a stream in a vessel. As used herein, stream is intended to comprise any body of flowing fluid.
Prior art clamp-on ultrasonic flow meters have been used to measure the velocity or volumetric flow rate of a liquid flowing through a pipe or other vessel. These flow meters are normally clamped onto the existing piping and do not interfere with operations since no special piping or isolation valves are needed. In these flow meters bursts of ultrasonic energy are transmitted across the pipe on a skewed path from a transmit transducer on one side of the pipe to a receive transducer on the opposite side. The ultrasonic pulses are speeded up slightly in the downstream direction by the component of flow velocity along their path and are retarded slightly when traveling in the upstream direction. By taking the difference in transit time of the sound bursts in each direction, a measure of the flow velocity is obtained. Generally, each of the two transducers can act either as a transmitter or receiver depending on the desired pulse direction. The longest transit time indicates the upstream direction and the direction of flow. These flow meters are limited to a relatively steep, short path (approximately 25 degrees off vertical) across the pipe. The vector component of flow along this path is greatly reduced because of its steep angle, and the short path reduces interaction time for flow effects to build up. Another detrimental effect of the short, steep beam employed by the prior art is that only a small portion of the flow is sampled and local disturbance can have a significant effect on the velocity sensed. In addition, the prior art ultrasonic flow meters have been limited to velocity measurements of liquids; such techniques have not been feasible in gas velocity measurements.
Therefore, it is an object of the present invention to provide a flow metering system for measuring the velocity of a fluid in a vessel or pipe by means of an extended path length to minimize the effect of local turbulence and maximize the vector component of flow of the stream.